1. Technical Field
The present invention relates to a two dimensional image forming device using a coherent light source as a light source. More particularly, the present invention relates to a two dimensional image forming device having means for reducing speckle noises appearing within the display.
2. Background Art
FIG. 16 schematically shows the configuration of a conventional laser display. Each of light beams from laser light sources 101a through 101c for three colors in RGB is expanded by a beam expander 102 and irradiated to a two dimensional spatial light modulation element 107 by a light integrator 103. The light integrator 103 is a fly's eye lens in which rectangular unit lenses are arrayed two dimensionally, and a light intensity distribution of substantially the Gaussian distribution forms an almost uniform rectangular shape on the two dimensional spatial light modulation element 107 so as to irradiate the two dimensional spatial light modulation element 107 at uniform intensity. A diffusing, plate 105 is disposed in front of the two dimensional spatial light modulation element 107 and rotated in-plane by a diffusing plate oscillation portion 112. The respective rays of light having passed through the corresponding two dimensional spatial light modulation elements 107 are combined by a dichroic prism 110 and projected onto a screen 108 by a projection lens 109 in the form of a full-color video.
A laser display as above is characterized in that a highly monochromatic laser light source is used as a light source. In a projector using a lamp, because light having a continuous spectrum of a lamp light source is separated to three colors in RGB, each ray of RGB light has a continuous spectrum distribution, which makes it impossible to display a pure single color. On the contrary, because the laser display uses a monochromatic light source, it is possible to display a sharp image having high color purity.
Incidentally, a problem with the display as above is so-called speckle noises occurring due to the use of a laser light source having high coherency as a light source. Speckle noises are minute irregular noises generated when laser light is scattered on the screen 108 as rays of scattering light from the respective portions on the screen 108 interfere with one another. In order to suppress the speckle noises, as has been described using FIG. 16, the conventionally proposed laser display has the configuration to oscillate the diffusing plate 105.
More specifically, the diffusing plate 105 is processed to have a ground glass surface, and confers random phase modulations to light coming incident thereon. A parallel beam coming incident on the diffusing plate 105 becomes scattering light scattered randomly within a specific angle. Light having passed through the diffusing plate 105 generates random speckle noises on the two dimensional spatial light modulation element 107. By oscillating the diffusing plate 105 in-plane, speckle noises on the two dimensional spatial light modulation element 107 change at a high speed, and speckle noises of an image projected on the screen 108 change at a high speed as well. When viewed by human eyes, speckle noises changing at a high speed are time-averaged and acknowledged as a noiseless smooth image.
The configuration described above, however, has an issue that part of light scattered on the diffusing plate 105 is lost. Hereinafter, how such an issue arises will be described in detail.
In order to suppress speckle noises more effectively, an angle of diffusion of light on the diffusing plate 105 is increased. In this instance, the angle of incidence of light irradiating the two dimensional spatial light modulation element 107 becomes larger, which increases the angle of incidence on the screen 108 of light irradiating the screen 108. Because speckle patterns generated from moment to moment depend on the angle of incidence on the screen 108, many more speckle patterns are generated as light goes incident at a larger angle, and they are averaged more effectively.
When the angle of diffusion is increased as descried above, light irradiating the outside of the image frame of the two dimensional spatial light modulation element 107 and light eclipsed at the pupil of the projection lens 109 are increased, which makes up a loss of light. By making a distance between the two dimensional light modulation element 107 and the diffusing plate 105 smaller, it is possible to lessen light irradiating the outside of the image frame of the two dimensional spatial light modulation element 107 on the one hand, but on the other hand, an image of the granular pattern of the diffusing plate 105 is formed on the screen 108 and noises other than the speckle noises are generated. Hence, it is necessary to secure a specific interval as the distance between the two dimensional spatial light modulation element 107 and the diffusing plate 105, and it is therefore impossible to eliminate light that deviates to the outside of the image frame of the two dimensional spatial light modulation element 107.
Meanwhile, it is possible to prevent a loss of light resulting from the eclipse on the projection lens 109 by setting the angle of diffusion on the diffusing plate 105 to or lower than the brightness (F value) of the projection lens 109. However, from the diffusing characteristics of the diffusing plate 105, an exiting light intensity distribution with respect to the angle of diffusion is normally a distribution like the Gaussian function. Hence, the eclipse on the projection lens 109 increases as the angle of diffusion is set larger.
In addition, because light intensity of a beam is made homogeneous using the light integrator 103 in the laser display described above, the light integrator 103 requires an optical path length to some extent, which makes the light integrator 103 longer. Also, because a beam coming incident on the light integrator 103 is expanded by the beam expander 102, the beam diameter becomes larger, too. This raises a need to use the beam expander 102 and the light integrator 103 having a large diameter. Consequently, the beam expander 102 and the light integrator 103 are increased in size and so is the optical system, which makes it difficult to reduce the laser display in size.
Patent Document 1: JP-A-7-297111